Method of making high strength



United States Patent 3,125,222 NETHGD OF MAKING HIGH STRENGTH MAGNESIUM EXTRUDEE;

George S. Foerster and Roy J. .lohnson, Midland, Mich,

assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Feb. 15, 1960, Ser. No. 8,493 4 Claims. (Cl. 207-) The invention relates to a method of extruding magnesium and magnesium-base alloys to produce extrudes having improved strength characteristics. The term magnesium will be used hereinafter to include both magnesium metal and magnesium-base alloys wherein the percentage of magnesium by weight is at least 75 percent.

. Magnesium possesses many properties that make it highly desirable for a wide number of uses. Among such properties are light weight, high thermal and electrical conductivity, thermal stability, fatigue resistance, and good natural resistance to corrosion in the absence of salt Water and acids, e.g., corrosion resistance to alkalis, hydrocarbons, aldehydes, alcohols, phenols, amines, esters, oils, alkali metal fluorides, arsenates, chromates and dichromates, and its response to surface treatment to improve its resistance to salt water and acids. Physical properties such as tensile strength, compressive yield strength, shear values, bearing strength, and moduli of elasticity and rigidity of cast or forged magnesium are satisfactory for a large number of uses. However, extruded magnesium produced by conventional extrusion methods possesses strength properties which are too low for acceptance for uses where it otherwise would preferably be the metal selected due to its light weight, good conductivity, fine-grained structure, and other desirable properties.

Attempts to produce magnesium extrudes which have desirable fine-grained strain-hardened structures and a satisfactory strength-weight ratio in comparison to other extruded common metals, e.g., aluminum, have not been suificiently successful to justify their adoption for certain specific uses. Among such attempts have been the introduction of solutionand precipitation-hardening, new magnesium-base alloys, and pellet extrusion. Cold working of magnesium extrudes heretofore, as by rolling and impact extrusion to increase strength properties has resulted in only a slight increase in yield strength accompanied by a substantial loss in ductility. Only the pellet extrusion has shown any significant improvement over theretofore known methods, i.e., ingot extrusion, but such improvement has been insufiicient to meet some of the strength property requirements for extruded magnesium.

A need, therefore, exists for an improved extrusion process whereby magnesium extrudes are produced which have fine-grained structure and sufliciently improved strength properties without substantial loss of ductility to permit the employment of such extrudes for uses which impose stresses thereon beyond that which magnesium extrudes produced by heretofore known methods have satisfactorily withstood.

The primary object of the invention is to provide such a process. The way by which the process is carried out to attain this and related objects is set forth in the ensuing description and is concisely defined in the appended claims.

The invention is predicated on the discovery that by die expressing a lubricated mass of magnesium or an alloy thereof containing at least 75 percent by weight of magnesium through a die, preferably conical shaped, ata sufficiently low reduction ratio, extrusion speed, and temperature to maintain the temperature during the extrusion below the recrystallization temperature of the metal being extruded, extrudes are produced which are fine grained,

retain a substantial amount of strain hardening, and possess strength properties combined with good ductility heretofore unequaled. By reduction ratio is meant the ratio of the cross-section of the billet cavity in the container or barrel of the extruder to the cross-section of the orifice formed by the die. This should be at least 2:1 but not so great as to tend to raise the temperature above the recrystallization temperature. The reduction ratio is less than that commonly employed in the conventional die expression of magnesium-base alloys and does not exceed 50:1. By extrusion speed, as used herein, is meant the rate at which the extruded product emerges from the die; it is not to be confused with the rate at which the ram travels in the extrusion chamber.

A preferred embodiment of the invention is predicated on the discovery that by re-extruding, in accordance with the foregoing method, the metal previously extruded below its recrystallization temperature, the strength properties thereof are further enhanced. Re-extruding in accordance with the foregoing provisions at least once and preferably two or more times is recommended, each extrusion being carried out in accordance with the aforementioned requirements. When re-extrusion is practiced, the already extruded article is either upset, that is com pacted in an extrusion press container having no exit, e.g., an extrusion press wherein the die opening is blocked off prior to a subsequent extrusion, the container having a larger cross-section than that of the article, or it is extruded through a die of somewhat smaller cross-section than that of the article when the article has about the same diameter of the container.

The discovery is a basis for marked departure from heretofore known methods of extruding magnesium and magnesium-base alloys. For example, the use of lubricants in such extrusion has not heretofore been satisfactory due, in part, to the observed tendency toward increased contamination at the temperatures employed in conventional practice; conical dies have not been exten sively used; and high reduction ratios have always been considered necessary and employed heretofore to achieve strain-hardening during extrusion; re-extrusion of alloys extruded by conventional practice has appeared to lessen rather than improve the strength properties of the extrudes; cold working or impact extrusion operations performed on the extrudes has resulted in a substantial decrease in elongation at the expense of a small increase in yield strength. The practice of the invention makes use of a novel combination of extrusion conditions, including conditions heretofore considered unfavorable, to provide a highly effective method of producing improved magnesium and magnesium alloy extrudes.

The invention, accordingly, consists essentially of extruding at least once, and preferably re-extruding one or more times, magnesium or a magnesium-base alloy metal, as in the form of ingots or compacted pellets, which have preferably been preheated and coated with lubricant, by extruding the metal through a die which is preferably conical-shaped, wherein the ratio of the cross-section of the extruder container to that of the die is between about 2:1 and 50:1, at a rate of extrusion which does not result in the metal in its passage from the container through the die attaining a temperature above the recrystallization temperature of the metal benig extruded. The preheating temperature, when employed, is preferably as high as can be used (to utilize as fully as possible the efiiciency of the extrusion press) without the metal in its passage to and through the die attaining the recrystallization temperature, having due regard for the reduction ratio and extrusion speed. The extrudes thus produced are fine grained and possess desirable strength properties, the presence of which is readily shown by higher values for such standard tests as tensile yield strength (T.Y.S.), tensile 3 strength (T.S.), and the compression yield strength (C.Y.S.).

The preferred temperature of the metal to employ in the extrusion press somewhat in accordance with the magnesiurnbase alloy being extruded. The lower extrusion temperatures employed accordingly to the invention are at least about 200 F. and more commonly at least 300 or 350 F. The extrusion temperatures recommended according to the invention are usually at (least about 50 and preferably between about 50 and 100 Fahrenheit degrees below the recrystallization temperature of the magnesium or magnesium-base alloy being extruded.

The rate of extrusion depends upon what temperature of extrusion is considered desirable to permit attainment of the best strength properties. The rate of extrusion is usually between about 0.5 and 5.0 feet per minute and more commonly between 1 and 2 feet IP61 minute.

Recrystallization is evidenced by a marked increase in the formation of discrete new grains followed by grain growth after replacement of distorted grains by new substantially stress-free grains.

Extrusion below the recrystallization temperature at appropriate speed and reduction ratio, according to the invention, using a lubricant, induces strain hardening and results in superior strength properties in the extrudes. In accordance with the invention, the recrystallization temperature is not exceeded and strain-hardening induced during extrusion is retained. A discussion of recrystallization and the significance thereof is set out in Metals Handbook, pages 259 to 263 (1948), published by the American Society for Metals (A.\S.M.), Cleveland, Ohio.

A satisfactory extrusion temperature for the practice of the invention may be ascertained by examining the crystal structure of extrudes at various temperature decrements, for example, decrements of 50 or 100 Fahrenheit degrees or evaluating the extruded alloy by the strength tests mentioned above. Extrusion thereafter may be then controlled readily by merely maintaining the rate of extrusion, reduction ratio, and container temperature and lubricating conditions within the range which had produced the satisiactory results. A convenient method of measuring the temperature of the extrude as it leaves the die consists of afiixing a brush or roller-type contact thermocouple at the die outlet, and reading the temperature registered thereon.

A lower temperature of extrusion, less likelihood of damage to equipment, and less resistance to advancing the stock through the die exist by the employment of a lubricant at the interface of the container wall and the metal passing thereth-rough. By virtue of the presence of the lubricant, the extrude is produced at a satisfactory rate and at a reduction ratio that permits extrusion without excessive temperature rise in the metal being extruded. The lubricant suitable for the practice of the invention may be one having satisfactory heat stability and one which lessens the iriction or drag on the extruding magnesium. Lubricants which are satisfactory in the practice of the invention include the following: (1) colloidal graphite preferably suspended in a volatile carrier, e.g., naphtha, mineral spirits or water, (2) polyethylene, (3) Teflon (polytetrafluoroethylene), and (4) silicate glasses such as those described in U.S. Patent 2,538,917.

A conical die is a die which has a converging or tapered passageway from the container of the extruder to the die orifice. It reduces the resistance from that offered by a hat die, i.e., one having at the entrance to the die orifice a plane face set at right angles to the direction of movement of the extrude through the orifice. By the employment of a conical die, a dead zone or relatively immobile zone of the metal being extruded, located between the die orifice and the confining wall of the extruder, which exists when a hat die is used, is substantially eliminated. The angle of the convergence is not highly critical but it is suggested that the included angle defined between the converging sides of the passageway through the die be between about and 160 and preferably about 120. A conical die, suitable (for use in the practice of the invention, is described in U.S. Patent 2,538,918. When a conventional or flat die is used with a lubricant, the lubricant is often at least partially stripped from the extruding metal "as it enters the die orifice and such stripped lubricant tends to accumulate in the dead zones about the die orifice or the flat die and be released sporadically in excess, thereby producing spots and blemishes on the extrude as well as contributing to erratic changes in extrusion rates and temperatures. The employment of the combination of lubricant and conical die within the temperature range of the invention largely eliminates [these undesirable eilects.

When magnesium pellets are employed as the starting metal in the practice of the invention, they are first compacted, usually by placing a charge thereof in an extruder container wherein the die exit is blocked and pressure and heat applied to the charge until a compact mass thereof is produced. The temperature employed is usually between about 500 and 800 F. and the total force applied on the mass between about 400 and 500 tons for a 3" diameter container. The compacted mass is thereatter ejected firom the container as :a billet.

A mass of pellets thus compacted, a pre-extruded billet, or a magnesium casting, to be extruded according to the invention, is then preheated, preferably to a temperature considerably above that of the container and die during subsequent extrusion. The pre-heat-ing temperature is usually from -200 Fahrenheit degrees above the extrusion press container temperature, the temperature of the container being controlled to impart a temperature to the metal therein which will result in a temperature at the die of less than the recrystallization temperature.

The following examples are illustrative oi the practice of the invention.

In the examples in which compacted magnesium pellets were employed, the pellets were prepared substantially as described in U.S. Patent 2,699,576. The extrusion apparatus was or conventional extrusion press design, except that a conical die was employed.

Mechanical properties of the extrudes produced in the examples were determined according to the following ASTM tests:

Property: ASTM Percent elongation (E) 1 E8-57T Tensile yield strength (TYS) 1 E8-57T Tensile strength (TS) E857T Compression yield strength (CYS) 2 E9-52T E8-57l is fully defined in ASTM Standards 1958 Part 3, pages 103 to 119. t 1E5l9-52T is fully defined in the same volume at pages 10 EXAMPLE 1 Magnesium alloy atomized pellets of ZK60, having the nominal composition of 5.5 percent Zn, 0.6 percent Zr, and balance Mg, were preheated to a temperature of 600 F. and then loaded into a 3-inch diameter container of an extrusion press having the die exit blocked by a blank insert. They were therein compacted under a total force of 500 tons into a billet. The container temperature was 550 F. The blank insert was then removed and the billet pushed out. The billet was then machined down to a size of 2% inches in diameter. It was then coated with a lubricant consisting of colloidal graphite suspended in mineral spirits, heated to 500 F., loaded into the 3-inch diameter container maintained at a temperature of 270 F., and extruded through a conical shaped die having a included angle therein, into a rectangular shape having a size of /8" by 2" which provided a reduction ratio of 9:1 at a rate of 1 foot per minute which resulted in an extrusion temperature below the recrystallization tempera tures.

For comparative purposes, Blank A was run wherein some of the same alloy pellets were compacted and preheated as described in Example 1 above and the billets thus made similarly machined. Therealfter, extnusions 6 and percent elongation of Blank B were measured. The results are set out in Table III as Blank B. The results obtained fior Blank A, as set out in Table I are again set out in Table III for convenient comparison.

were made by placing a billet so made in the 3-inch diam- 5 A further example of the invention was then run also eter container having a temperature of 730 F. and exemploying Z1460 magnesium alloy pellets. In this examtruding the billet through the by 2" die at a rate of l ple the pellets were compacted in a 4-inch diameter confoot per minute. The temperature of extrusion was above tainer in a similar manner to that followed in the examthe recrystallization temperature as shown by exarninaples above, removed from the container, and machined tion of asample of the extrude. Strength properties of the to a 3 As-inch diameter billet. The billet thus made was extrudes produced by both the example and blank were lubricated with the graphite lubricant employed in the determined according to the ASTM methods designated examples above, preheated to a temperature of 350 F.,

herein above. The results are set out in Table 1 below. and thereafter loaded into the 4-inch diameter container Table I having a temperature of 300 F. and extruded through a 2% inch. diameter die (about a 3.5 :1 extrusion reduction Strengthm Thousands ratio) at a rate of 0.5 foot per minute. The extruded Ext-rude, f Pounds/Square Inch rod thus produced, was pickled in an aqueous bath com- Test Oolntainer Per gent prising NaNO and CH COOH in accordance with US.

5 3: TYS CYS TS Patent 2,607,739 to remove the lubricant. Compression yield strength and percent elongation were measured on Bank A 730 1g 36 36 43 the extrude thus produced. The results are set out as Emmple 1 270 12 56 56 64 Example 9 in Table III.

The extrude thus made was then upset by placing it Reference to Table I shows that the strength propin a 3-inch diameter container having the exit blocked erties (TYS, CYS, and TS) obtained by the tests on Blank and pressing it into again a 3-inch diameter billet. The A, which was produced at an extrusion temperature above billet thus made was again lubricated with the graphite, the recrystallization temperature, are definitely inferior preheated again to 350 F., and placed in a 3-inch dito the results obtained on Example 1, which was produced ameter container having a temperature of 310 F. It at a temperature of extrusion below the recrystallization was re-extruded through a 1-inch diameter die, a reductemperature of the alloy, both the blank and the example tion ratio of 9: 1, at an increased extrusion rate of 1 foot being made at the same reduction ratio and extrusion per minute. The compression yield strength and percent speed. elongation were measured on the extrude thus produced EXAMPLES 2-8 by re-extruding in accordance with the invention. The

Additional examples were run wherein different magi g if set 5 i exnesium-base alloy pellets were compacted and extruded 0 ru es Us g recrys a ma i with a lubricant below the recrystallization temperature peratur; as reqlilre i i i Possessed Improve at extrusion rates and reduction ratios in accordance with 'Strengt propfimes an goo uctlhty' the invention and the physical properties of the extrudes Table III so produced ascertained. Billets were thus produced and 4G subjected to the same preheating step as that employed in Extruder Carmina Percent C pr Example 1. The preheated billet was then coated with the Test Temperature m c E 1%;? lubricant employed in Example 1 and extruded at a reduction ratio of 9:1 at a rate of extrusion Olf 1 foot Blank A 73g 18 36 per minute. The extrndes thus produced below the re- BlankB (Blank A stock, 001d 4 4 crystallization temperature as required by the invention a E g N &? 15 52 possessed improved strength properties and good ductility. Example 10 310 9 6 Results :of the strength tests are set out in Table II.

Table II 'Extruder Strength in Thousands Container of Pounds] Square Inch Example Nominal Alloy Composition Section Temp. Perlrent m TYS oYs TS 2 3.(Mrl,0.4 Mn, 1.0 Zn, balance x2-.... 410 12 36 38 47 a Liiir l; 1.0 Zn, 1.0 Mn, balance %"x2 410 s 37 3e 47 x 2"-.- 390 e 39 37 39 x 2" 400 7 39 42 49 x 2-.. 310 12 4s 44 50 1" dameter 350 10 5s 56 Bab 310 s 61 5s 65 1 Misch metal.

It is clear from the results obtained in the tests set out The results of Table III show that the physical properin'Table II that magnesium magnesium-base alloys ties of the magnesium extrude produced according to the eXl-Yuded?10C0Tdingt0'he invention Possess excellflljlt Y invention are superior to those produced by convenoompmsslon and 5113116 Strength and good dulcmhtytional practice. They further show that attempts to im- EXAM-PLES 9 AND 1 prove compression yield strength in conventionally ex- Blank A of Table -I which consisted of ZK60 magnesiuum alloy pellets, compacted and extruded according to conventional practice, were cold worked until cracking was observed. Blank A, thus altered by cold Working truded magnesium alloys by cold rolling not only failed to improve the compression yield strength more than a small percent but so impaired the ductility, as evidenced by the markedly lowered percent elongation, as to render was designated Blank B. The compression yield strength such treatment inadvisable for a large number of purposes. In contrast to the blanks, extrudes made according to the invention not only showed definite improved compression yield strength when extruded once (Example 9) and a pronounced improvement therein when re-extruded (Example 10) but retained much greater ductility as evidence by the higher final percent elongation values.

pass through the extruder was at a rate of 1 foot per minute and at the container temperatures set out in Table V. The first run, designated Blank C, was made above the recrystallization temperature, the other runs designated, Examples 14-20 were run below the recrystallization tempenature and illustrate the invention.

Table V Reduction Ratio Extruder Strength in Thousands (Diameter of Con- Total Re- Passes Speed of Pounds Per Test Container to tainer duction Made in Feet/ Percent Square Inch Diameter of Die) Temp. Ratio Thru Minute E inches of F Extruder TYS OYS TS Blank O 700 1 64:1 1 1 10 35 35 45 Example 14.'.. 400 2 4:1 1 1 5 53 42 61 Example 15... 400 16:1 2 1 9 61 53 68 Example 16.-- 400 64:1 8 1 9 58 58 64 Example 17... 400 256:1 4 1 3 58 58 60 Example 18--. 350 16:1 2 1 8 62 54 71 Example 19-.. 350 64:1 8 l 7 65 64 75 Example 20... 350 256:1 4 l 2 71 72 75 1 This represents a single pass for purposes of comparison. 1 Each pass through the extruder in accordance with the invention was at a. reductlon ration of 4:1.

EXAMPLES 11-13 Another series of examples was run at extrusion temperature below the recrystallization temperatures according to the invention, employing a magnesium alloy having a nominal composition of 6 percent Zn, 2 percent rare earth metals, 0.6 percent Zr, and balance Mg. The compacting step, the lubrication, and the first extrusion step were similar to that of Example but were done at the temperature set forth in Table IV. The temperature of the extruder container, the rate of extrusion, the extrusion reduction ratio, and the physical test values obtained on the extrudes thus made are also set forth in Table IV below. The extrudes thus produced below the recrystallization temperature as required by the invention possessed improved strength properties and good ductility.

It is readily seen by referring to the results of Table V that when the temperature of extrusion exceeds the recrystallization temperature of the alloy being extruded, the tensile yield strength, the compression yield strength, and the tensile strength are all lessened very definitely. This is particularly noticeable since the reduction ratio according to the comparative run or blank was 64:1, a greater reduction ratio (and therefore evidence of greater metal working), than that employed in any single pass made according to the invention.

The practice of the invention not only permits the desirable practice of either compacting pellets and then using the compacted pellets as stock for extrudes or using cast or pre-extruded billets as stock for extrudes by first extruding the stock into a convenient oversized extrude and subsequently passing the first extrude thus made 1 A rate at which the extruded stock emerges from the die.

An examination of the results obtained and set out in Table IV are conclusive of the merits of a double extrusion process in accordance with the practice of the invention at varying container temperatures, speeds, and extrusion reduction ratios so long as the temperature of the alloy being extruded is below the recrystallization temperature thereof.

EXAMPLES 14-20 To show further the advantages of the invention over conventional extrusion operations, a series of test runs were made wherein magnesium-base alloy pellets composed largely of 0.7 percent Zr, 6.0 percent Zn, 1.6 percent misch metal, and balance magnesium were compacted as in the above examples, preheated to 500 F. under pressure, removed from the compression chamber, lubricated as above described, and given a first extrusion employing a 3-inch diameter container and a conical die having an exit of 1 /2 inches to produce a round rod. The thus extruded rod was cut into 8-inch lengths and the lengths thus made lubricated and re-extruded successively at a 4:1 extrusion reduction ratio for each pass a sufficient number of times to give the total or cumulative extrusion reduction ratios set out in Table V below. Each through a die of desired size but also affords the added advantage of enhancing the strength properties of the extrude thus made. The invention, therefore, .not only provides extrudes of high strength properties, with less sacrifice in ductility, useful for subsequent extrusion but also those having a fine-grained strain hardened structure useful as stock for subsequent forging or rolling into high strength plate or sheet.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. The method of direct extrusion of metal during which the metal undergoes cold working, said metal being composed of at least 75 percent by weight magnesium, to produce extrudes having superior strength properties, consisting of extruding said metal from the container of an extrusion press through the die orifice thereof wherein the extrusion reduction ratio of the cross-section of said container to that of the die orifice is between 2:1 and 50:1, lubricating the interface between said metal and the contacting surfaces of said container and die, and maintaining the temperature of the extrude being produced at not less than about 300 F. and below the recrystallization temperature of the extruded metal as measured as the extrude leaves the die orifice, and thereafter upsetting the extrude so produced whereby the crosssectional area of the extrude is increased and the thus upset prior extrude of increased cross-sectional area is re-extruded, to produce an extrude which retains a substantial amount of the strain hardening induced therein during the extrusion operation.

2. The method according to claim 1 wherein the extrusion is preceded by preheating the metal to be extruded at a temperature in excess of 300 F. and below the recrystallization temperature of the metal to be extruded, said preceding preheating temperature being above the subsequent extrusion temperature.

3. The method according to claim 1 wherein the extrusion is carried out employing an extrusion press provided with a tapered approach to said die orifice.

4. The method according to claim 3 wherein the included angle of said tapered approach is between about 90 and about 150".

References Cited in the file of this patent UNITED STATES PATENTS 1,594,347 Bakken Aug. 3, 1926 1,946,121 Wood Feb. 6, 1934 2,218,459 Singer Oct. 15, 1940 2,371,716 Snell Mar. 20, 1945 10 2,538,917 Sejournet et a1. Jan. 23, 1951 2,578,585 Orozco et a1 Dec. 11, 1951 2,589,881 Sims et a1. Mar. 18, 1952 2,630,623 Chisholm et a1. Mar. 10, 1953 2,753,994 Bridge July 10, 1956 2,755,544 Moore July 24, 1956 2,755,546 Moore July 24, 1956 2,887,472 Fotis May 19, 1959 2,893,554 Sejournet et al. July 7, 1959 3,046,304 Haszeldine July 24, 1962 FOREIGN PATENTS 209,338 Australia May 15, 1957 745,006 Great Britain Feb. 15, 1956 779,603 Great Britain July 24, 1957 564,081 Great Britain Sept. 12, 1944- OTHER REFERENCES The Metallurgy of Deep Drawing and Pressing, by

20 J. Dudley Jevons, 2nd ed., 1940, pub. by John Wiley 25 of Magnesium relied upon. 

1. THE METHOD OF DIRECT EXTRUSION OF METAL DURING WHICH THE METAL UNDERGOES COLD OWRKING, SAID METAL BEING COMPOSED OF AT LEAST 75 PERCENT BY WEIGHT MAGNESIUM, TO PRODUCE EXTRUDES HAVING SUPERIOR STRENGTH PROPERTIES, CONSISTING OF EXTRUDING SAID METAL FROM THE CONTAINER OF AN EXTRUSION PRESS THROUGH THE DIE ORIFICE THEREOF WHEREIN THE EXTRUSION REDUCTION RATIO OF THE CORSS-SECTION OF SAID CONTAINER TO THAT OF THE DIE ORIFICE IS BETWEEN 2:1 AND 50:1, LUBRICATING THE INTERFACE BETWEEN SAID METAL AND THE CONTACTING SURFACES OF SAID CONTAINER AND DIE, AND MAINTAINING THE TEMPERATURE OF THE EXTRUDE BEING PRODUCED AT NOT LESS THAN ABOUT 300*F. AND BELOW THE RECRYSTALLIZATION TEMPERATURE OF THE EXTRUDED METAL AS MEASURED AS THE EXTRUDE LEAVES THE DIE ORIFICE, AND THEREAFTER UPSETTING THE EXTRUDE SO PRODUCED WHEREBY THE CROSSSECTIONAL AREA OF THE EXTRUDE IS INCREASED AND THE THUS UPSET PRIOR EXTRUDE OF INCREASED CROSS-SECTIONAL AREA IS RE-EXTRUDED, TO PRODUCE AND EXTRUDE WHICH RETAINS A SUBSTANTIAL AMOUNT OF THE STRAIN HARDENING INDUCED THEREIN DURING THE EXTRUSION OPERATION. 